1. Field of the Invention
This invention relates to a scanning probe microscope and an information recording and/or reproducing apparatus using the same.
2. Related Background Art
In recent years, a scanning tunnel microscope (hereinafter abbreviated as STM) has been developed [G. Binning et al., Phys. Rev. Lett., 49, 57 (1982)] and it has become possible to observe the real spatial image of the surface of a conductor at a high resolution of atomic order irrespective of single crystal or amorphous materials. The STM utilizes the fact that a tunnel current flows when a voltage is applied between a metallic probe and the surface of a conductor and the distance therebetween is approximated to the order of 1 nm. The tunnel current is very sensitive to the distance between the two. The probe is scanned on the surface of the conductor while the distance between the two is controlled so as to keep this tunnel current constant, whereby the surface shape of the conductor is formed from the control signal of the distance. At this time, the resolution in the in-plane direction reaches the order of 0.1 nm. It has recently been reported that even the observation of the atomic images or the molecular images of rare gas atoms or organic molecules adsorbed to the surface of the conductor is possible. These results are interpreted that the STM also detects the information regarding the interaction between an electron cloud on the tip end of the probe and an electron cloud on the surface of the sample.
The STM has an advantage that it enables observation to be effected by low electric power without imparting any damage by the electric current to the sample. Further, it can be operated in the atmosphere and can be used for various materials. Therefore, its application not only to surface observation but also to wide fields is expected. For example, its application to the minute working of surfaces and high density information recording has been proposed. Furthermore, there have been developed microscopes to which the art of STM is applied and by which interactions of various types between the surface of a sample and the tip end of the probe are detected to thereby enable the state of the surface to be observed, and these, including the STM, are known generically as scanning probe microscopes.
When effecting the surface observation by the STM, a bias voltage is applied between the sample and the probe to send an electric current. Where the sample is a conductor, the bias voltage can be set to a sufficiently low voltage. However, where the sample is a semiconductor, it is necessary to set the bias voltage to a relatively high level (1-3 V) and position the Fermi level of the probe in the conduction band or the valence band of the semiconductive sample. Also when observing an organic molecule layer formed on the surface of the conductor, the bias voltage must be set such that an electron cloud resulting from the organic molecule and an electron cloud at the tip end of the probe interact. The value of this bias voltage varies depending on the interaction between the surface of the conductor and the organic molecule, but generally it is often a high voltage of 1-2 V.
Such a great bias voltage is a match for the coupling energy of molecules or solids. Also, by a DC bias voltage being applied, particularly the organic molecule layer is steadily placed in its polarized state during the STM observation. Further, in the STM, the probe and the sample are very close to each other and therefore, a very high electric field is created between the tip end of the probe and the surface of the sample by the high bias voltage. If the high bias voltage continues to be applied for a long time in case of the surface observation, the state of the surface to be observed may gradually change. The state of the tip end of the probe may also change, and there is the possibility that an accurate surface image is not formed.
Such a situation, however, is not limited to the STM, but in a recording apparatus or a reproducing apparatus utilizing the STM as well, the surface of a recording medium may be changed in its quality by recording or reproduction being repeated, and this will pose a problem in effecting stable recording or reproduction.